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I have a Quectel M72FA GSM module, brand new one. I have just soldered it onto my PCB and found that: there is a short-circuit between the GND and RF_ANT pins.

This is the pinout:
GSM pinout

I have checked the soldering with a magnifying glass, it looks OK the pins are not connected by the solder. It can be seen in the following picture:

PCB

Here is the functional diagram of the module: enter image description here

The functional diagram of the module shows, there is an ESD protection diode between the RF line and the GND.

  1. Can this short-circuit be caused by the malfunction of this ESD protection part, (malfunction of the module)?
  2. I am measuring 1.1 Ω resistance between the RF and GND pins, it can not be normal, can it be?
  3. All in all, I have checked the soldering and the PCB trace of the RF_ANT pin, both seems to be good. What else should I check before suspecting that my brand new module is a factory defective piece?

Antenna reference design from datasheet (it is all about the antenna design, beside this and the functional diagram there is no other information about the RF design of the module in the datasheet): enter image description here

M72 provides an RF antenna pad for antenna connection. The RF trace in host PCB connected to the module RF antenna pad should be coplanar waveguide line or micro-strip line, whose characteristic resistance should be close to 50 Ω. M72 comes with two grounding pads which are next to the antenna pad in order to give a better grounding.

I use the default setup, so there are no additional capacitors to the GND. I have an antenna like this. I have not connected it yet, as the SMA connector is not soldered (see picture above) because it would make more difficult to solder the module.

Meanwhile, I have contacted the manufacturer about the issue and they have confirmed that; it is a normal behaviour.

Regarding you question of RF_IN short with GND, it’s normal. You can ignore this point.

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  • \$\begingroup\$ The short could be on the PCB, not necessarily right between the pins. Desoldering it and checking again could be a good idea. \$\endgroup\$ – Eugene Sh. May 1 '15 at 20:32
  • \$\begingroup\$ The whole trace to the RF_ANT pin (quarter-circle shaped one) is visible on the picture above and it can be seen that there is no unwanted connection to the GND. Though the pad of the RF pin is not visible because of the module, that one worth a check. \$\endgroup\$ – Bence Kaulics May 1 '15 at 20:43
  • \$\begingroup\$ Does the circuit work? If yes, then what is your problem? \$\endgroup\$ – Andy aka May 1 '15 at 20:53
  • \$\begingroup\$ I have started the soldering of the module with these 7 pins, then checked for short-circuits just to be sure before moving on to the other pins. So I did not test if the module is working, as I have not finished the soldering yet (software is missing too). I did not want to risk damaging it so first I want to find out that it is a problem or an expected/normal behavior. \$\endgroup\$ – Bence Kaulics May 1 '15 at 21:01
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If you measured the resistance between the feed conductors of a dipole antenna you'd see an open circuit - this doesn't mean that something has gone open circuit.

If you measured the resistance of a loop antenna it would read maybe less than one ohm and this doesn't mean that there is a short circuit.

If you measured the input impedance of a dipole at it's resonant frequency it would be 73 ohms but there is no resistance in the wires to speak of.

Antennas cannot be judged to be working (or not working) with a simple DC test unless the type of antenna is known. I'm not about to read thru all 60 odd pages of the document linked in your question so if you want to know the real answer please find the relevent sections and maybe post the details.

Because this is also a radio module i.e. will have an RF output fed via LC filters to possibly a patch antenna, the inductors that shape the frequency response may be grounded and thus give the impression that the pins are faultily connected to ground.

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  • \$\begingroup\$ I have edited the question and added all parts of the datasheet about the antenna, except the output power, sensitivity, operating frequencies. \$\endgroup\$ – Bence Kaulics May 1 '15 at 22:24
  • \$\begingroup\$ Internal to the module, to restrict the spectral shape outputted there are likely to be harmonic filters and these are likely to comprise inductors to ground. They would show up as low ohmic connections to ground. Try searching for images of antenna filters on transmitters and you'll see what I mean. \$\endgroup\$ – Andy aka May 1 '15 at 22:32
  • \$\begingroup\$ I think I got the point, I will take your advice. Thank you very much. \$\endgroup\$ – Bence Kaulics May 1 '15 at 22:37
  • \$\begingroup\$ I see, it must be something like this, it says it is a tunable matching circuit for multi-band GSM applications. My module is a Dual-band GSM/GPRS engine so it can contain something like that circuit. \$\endgroup\$ – Bence Kaulics May 2 '15 at 13:35
  • \$\begingroup\$ Yes, something like that would be typical of a low power transmit stage and maybe more for a more powerful transmit stage. \$\endgroup\$ – Andy aka May 2 '15 at 14:34

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